Fix ICE in pass_rpad.

Besides conversion instructions, pass_rpad also handles scalar
sqrt/rsqrt/rcp/round instructions, while r12-3614 should only want to
handle conversion instructions, so fix it.

gcc/ChangeLog:

	* config/i386/i386-features.c (remove_partial_avx_dependency):
	Restrict TARGET_USE_VECTOR_FP_CONVERTS and
	TARGET_USE_VECTOR_CONVERTS to conversion instructions only.